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Is there an easy way to sort a list by two criteria?

Hello!
I am a newbie in Python. Recently, I get stuck with the problem of
sorting by two criteria. In brief, I have a two-dimensional list (for
a table or a matrix). Now, I need to sort by two columns, but I cannot
figure out how to do that. I read somewhere that it is possible to do:
>>table.sort().sort()
but it does not work.
Can anyone help me with this?
PS: I am using Python under Ubuntu 6.06.

Best,
PM
Feb 10 '08 #1
5 7501
neocortex wrote:
Hello!
I am a newbie in Python. Recently, I get stuck with the problem of
sorting by two criteria. In brief, I have a two-dimensional list (for
a table or a matrix). Now, I need to sort by two columns, but I cannot
figure out how to do that. I read somewhere that it is possible to do:
>>>table.sort().sort()
but it does not work.
Can anyone help me with this?
PS: I am using Python under Ubuntu 6.06.
You can specify an arbitrary comparison function with the cmp key to
sort. IOW, use table.sort(cmp=f), where f is defined to compare table
entries (rows?) by whichever criteria are required.
Feb 10 '08 #2
On Sat, 09 Feb 2008 18:05:14 -0800, neocortex wrote:
Hello!
I am a newbie in Python. Recently, I get stuck with the problem of
sorting by two criteria. In brief, I have a two-dimensional list (for a
table or a matrix). Now, I need to sort by two columns, but I cannot
figure out how to do that.

Can you give a (small, simple) example of your data, and what you expect
if you sort it successfully?

I read somewhere that it is possible to do:
>table.sort().sort()
but it does not work.
No it doesn't, because the sort() method returns None, and None doesn't
have a sort() method of its own.

table.sort() will sort table in place, so you need something like this:
>>table.sort()
table.sort()
except naturally that just does the exact same sort twice in a row, which
is silly. So what you actually need is something like this:
>>table.sort(magic goes here)
table.sort(different magic goes here)
where the first piece of magic tells Python to sort by the first column,
and the second by the second column. But to do that, we need to know more
about how you're setting up the table.

I'm *guessing* that you probably have something like this:
table = [ ['fred', 35, 8], # name, age, score
['bill', 29, 8],
['betty', 30, 9],
['morris', 17, 4],
['catherine', 23, 6],
['anna', 45, 8],
['george', 19, 5],
['tanya', 27, 7],
]
Now let's sort it:

>>from operator import itemgetter
import pprint
table.sort(key=itemgetter(0)) # sort by name
table.sort(key=itemgetter(2)) # sort by score
pprint.pprint(table)
[['morris', 17, 4],
['george', 19, 5],
['catherine', 23, 6],
['tanya', 27, 7],
['anna', 45, 8],
['bill', 29, 8],
['fred', 35, 8],
['betty', 30, 9]]

Does this help?

--
Steven
Feb 10 '08 #3
Hello!
Thank you all, so much! Now I can do double-criteria sort in at least
three ways. More than I have expected.

Best,
PM
Feb 10 '08 #4
[repost]

Duncan Booth:
>from operator import itemgetter
lst = [(1,2,4),(3,2,1),(2,2,2),(2,1,4),(2,4,1)]
lst.sort(key=itemgetter(1))
lst.sort(key=itemgetter(2))
lst
[(3, 2, 1), (2, 4, 1), (2, 2, 2), (2, 1, 4), (1, 2, 4)]
A little known thing from Python 2.5:
>>from operator import itemgetter
lst = [(1,2,4),(3,2,1),(2,2,2),(2,1,4),(2,4,1)]
sorted(lst, key=itemgetter(2, 1))
[(3, 2, 1), (2, 4, 1), (2, 2, 2), (2, 1, 4), (1, 2, 4)]

Bye,
bearophile
Feb 11 '08 #5
On Feb 11, 10:47 am, bearophileH...@lycos.com wrote:
[...]
A little known thing from Python 2.5:
[...]
>sorted(lst, key=itemgetter(2, 1))
Cute, thanks :-)

--bjorn
Feb 11 '08 #6

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